1. Field of the Invention
The present invention relates to an integrating photometer for measuring a total flux of light generated from a light source to be measured, and a method for measuring the total flux of light through use of the same. More particularly, the present invention relates to a configuration suitable for measurement of a total flux of light from a surface light source.
2. Description of the Background Art
As a typical device for measuring a total flux of light from a light source, there has been known a spherical integrating photometer including an integrating sphere. The integrating sphere has an inner face to which a diffuse reflection material (e.g., barium sulfate, PTFE (polytetrafluoroethylene)) is applied. A light source to be measured is installed in the integrating sphere and a flux of light generated from the light source to be measured is reflected repeatedly by the inner face of the integrating sphere, in order to render an illuminance at the inner face of the integrating sphere uniform. The spherical integrating photometer utilizes a characteristic that this uniform illuminance is proportional to a total flux of light from the light source to be measured. In general, a total flux of light measured by such a spherical integrating photometer takes a relative value; therefore, an absolute value of the total flux of light from the light source to be measured is obtained by comparison with a result of measurement (a standard value) in a case where a total flux of light generated from a known total-flux-of-light standard light source is used.
The spherical integrating photometer described above has the following disadvantage. That is, it is inevitable that light absorption occurs at a supporting structure for installing the light source to be measured in the integrating sphere, a baffle for preventing light generated from the light source to be measured from entering directly into a light receiving unit for measuring an illuminance, and the like. In addition, the light source to be measured also absorbs light.
As one of countermeasures against such light absorption, use of a coefficient for correcting self-absorption by the light source to be measured is disclosed in “JIS C-8152: Measuring methods of white light emitting diode for general lighting”. This self-absorption correcting coefficient is calculated as follows. That is, a self-absorption measuring light source (typically, a white LED (Light Emitting Diode)) is provided for preventing generated light from entering directly into the light receiving unit. Then, an output from the light receiving unit, which is generated in accordance with a flux of light from the self-absorption measuring light source in a case where a light source to be measured is installed in the integrating sphere, is compared with an output from the light receiving unit, which is generated in accordance with a flux of light from the self-absorption measuring light source in a case where the light source to be measured is not installed in the integrating sphere, to calculate a ratio therebetween. However, this method fails to correct light absorption by a supporting structure, a baffle and the like.
In order to avoid an influence of the light absorption by the supporting structure and the like, there has been proposed a hemispherical integrating photometer as disclosed in Japan Patent Laying-Open No. 06-167388. In place of the integrating sphere described above, this hemispherical integrating photometer includes an integrating hemisphere having a hemispherical inner wall to which a light diffusing material is applied, and a planar mirror provided such that a mirror face thereof covers an opening of the integrating hemisphere. Herein, a light source to be measured is installed at a center of the planar mirror such that a center thereof is aligned with a center of curvature of an inner hemisphere of the integrating hemisphere.
With this configuration, the light source to be measured and a virtual image of the light source to be measured, which is formed by the planar mirror, exist in an integrating sphere (a space formed by a combination of the integrating hemisphere and the virtual image of the integrating hemisphere). That is, the planar mirror and the integrating hemisphere provide with a state as if the light source to be measured lights up without provision of a supporting structure for fixing the light source at the space in the integrating sphere, leading to avoidance of light absorption by the supporting structure such as a lighting jig.
However, the light absorption by the light source to be measured is inevitable even when the hemispherical integrating photometer described above is used. In the case of using the hemispherical integrating photometer, particularly, the light source to be measured is attached on the planar mirror side; therefore, a surface light source having a relatively large light emitting area can be used as the light source to be measured. In such a case, occasionally, an amount of the light absorption by the light source to be measured is not negligible.
If the self-absorption measuring light source described above is provided for correcting the light absorption by the light source to be measured, there may arise a new problem. That is, if the self-absorption measuring light source is provided, a virtual image of the self-absorption measuring light source also exists in the integrating sphere. As described above, the baffle or the like must be provided for preventing the light from the self-absorption measuring light source from entering directly into the light receiving unit. However, the baffle becomes relatively large in size in order to avoid the direct entering of the light from the self-absorption measuring light source and the virtual image thereof into the light receiving unit. Consequently, unevenness in illuminance occurs in the integrating sphere by the baffle and the self-absorption measuring light source, resulting in a new measurement error due to the unevenness in illuminance.